Iain Armitage, Lance Barber and Montana Jordan will appear as their spinoff characters in a December 'Big Bang' episode.

The space-time continuum on The Big Bang Theory is about to be disrupted.

A December episode of the CBS comedy, which is in its final season, will feature a brief crossover with its spinoff, Young Sheldon. Three of that show's stars — Iain Armitage, Lance Barber and Montana Jordan — will make cameos as their characters.

CBS and producer Warner Bros. TV are keeping details under wraps, but considering Young Sheldon takes place almost 30 years prior to the present-day setting of Big Bang, some sort of explanation, be it a dream sequence or a wormhole, might be necessary.

Armitage plays the young version of Sheldon Cooper (Jim Parsons), who starts attending high school at age 9 in his East Texas hometown. Barber plays Sheldon's dad, George, and Jordan is Sheldon's older brother, Georgie (Jerry O'Connell has played grown-up George Jr. in a few Big Bang episodes).

Big Bang Theory has featured adult versions of Young Sheldon characters before, including O'Connell's adult Georgie and Robert Wu as the grown-up Tam, Sheldon's childhood friend. Laurie Metcalf played Sheldon's mom, Mary, on Big Bang for years before Young Sheldon cast Metcalf's real-life daughter, Zoe Perry, as the younger version of the character in the spinoff.

Parsons narrates Young Sheldon and is an executive producer of the spinoff.